State-of-the-art single junction solar arrays as well as concentrators using single junction solar cells utilize only a limited portion of the available solar spectrum, thereby wasting the remainder of available energy outside of their limited spectral response. The limitation is caused mainly by two basic “photon loss” mechanisms within the cells, namely, (1) loss by longer wavelengths and (2) loss by excess energy of photons. In the former case, photons with energy smaller than the “energy bandgap” or “forbidden gap” Eg (direct bandgap semiconductor) or Eg−Ephonon (indirect bandgap semiconductors where Ephonon is the phonon quantum of energy) cannot contribute to the creation of electron-hole pairs. In the tatter case, in the spectrum range of interest, one photon generates only one electron-hole pair. The rest of the energy larger than the bandgap is dissipated as heat. Photons with energy hν≧Eg thus can only use a portion of Eg of their energy for generation of electron-hole pairs. The excess energy raises the temperature of the solar cell and degrades its performance. Thus, even high quality cells with excellent quantum efficiencies, such as GaAs, exhibit relatively modest conversion efficiencies since they cannot respond to more than a relatively small portion of the incident spectrum.
One way of circumventing this limitation is the use of two or more different bandgap cells that are stacked, or monolithically grown, in a vertical manner. Such a multi-junction (MJ) system with appropriately chosen bandgaps can span a significantly greater portion of the incident solar spectrum than achievable with single-junction cell systems. Such multi-junction solar cells are well-known. For example, three-junction cells have been devised that can control a relatively larger portion of the solar spectrum, and are further described below. Because of their potential for very high efficiencies, MJ cells have enjoyed increased interest over the last two decades.
At a NCPV (National Center for Photovoltaics) meeting in Denver, Colo. on Apr. 16-19, 2000, it was reported that triple junction GaInP2/GaAs/Ge concentrator cells developed by NREL (National Research Energy Laboratory) and Spectrolab have achieved 32.3% at 47 suns and 29% at 300 suns (AM1.5, 25C), with an obvious drop of 3.3% (absolute) or 10.2% (relative), indicating one of the many limitations of MJ concentrator systems at higher concentrations. It should be kept in mind that the above-mentioned encouraging achievement with a pulsed solar simulator does not represent a real life situation. Under actual operating conditions, the MJ concentrator system performance can drop more than 12 to 15% (absolute) against the bare cell performance and defeat the use of high efficiency MJ cells. Some of the major concentration-related performance losses in MJ cells are caused by the following shortcomings: absorption of light in the top cells, chromatic aberrations caused by the concentrator optics, flux non-uniformity on the cells, limited heat removal from the top cells, current limitation in the cells, series resistance, shadowing losses due to finger contacts on the cells, and limited acceptance angle for photon incidence on the cells. Most of these limiting factors apply to all conventional concentrator types based on a variety of cells. MJ cells, however, are more vulnerable to most of these performance-limiting factors.
The relative deterioration of MJ cells becomes worse as the number of junctions increases. Several authors in the field have predicted that for vertically stacked or monolithically-grown systems, limited improvements are expected beyond triple-junction cells. A recent press release by Boeing (Spectrolab) on Aug. 15, 2001, confirmed that a triple junction cell developed by Spectrolab and NREL has reached a conversion efficiency of 34% (a world record at that time) at 400×. That appears to be very much the limit of three-junction cells. Four-junction cells are predicted to be able to reach upper 30% and lower 40% efficiencies. Theoretical studies have shown that to achieve this kind of efficiency level, a four-junction cell system requires a 1 eV bandgap III-V cell that meets all requirements including: optical, thermal, and electronic issues involved. In spite of extensive efforts, this material remains elusive.
Another shortcoming of the monolithic MJ cells lies in the limitation of complementary bandgap cell materials with matching lattices. In vertically-grown MJ cells, all the adjacent “sub-cells” must have matching or slightly mis-matching lattices for proper performance. Thus, even the best bandgap matched sub-cell cannot result in a multi-junction cell if their lattices mis-match. This requirement narrows down significantly the available set of sub-cells that could be used.
These apparent limitations represent a formidable bottleneck in the development of high and very high efficiency (and therefore cost-competitive) concentrator systems in the near future. According to analytical studies, ideal four bandgap cell systems utilizing a new 1 eV material can improve the solar to electricity conversion efficiency over 48% at 500 suns. Even at a cost of $250/Watt for such a system, the effective cell system cost for a 500× flux concentrator can be as low as $0.50/Watt. At this cost level, the concentrators would be ahead of the long range goals of the Department of Energy for PV flat plate technology (installed system cost of $1.00/Watt to $1.50/Watt by the year 2030), if the balance of concentrator system could be built for $0.50/m2 to $1.00/m2. Thus, very high cell and system efficiencies are paramount to achieve the long term cost goals for photovoltaics in general.
In the late 1990s, NASA and JPL scientists proposed an alternative technique, called “Rainbow”, to circumvent the problems of vertical MJ systems and improve the performance of multi bandgap cell systems. Their method is to split the solar spectrum into several frequency bands and focus each frequency band onto separate cells with corresponding energy bandgaps. The Rainbow multi-bandgap system represents a combination of solar cells, concentrators, and beam splitters. The use of separate discrete cells offers the widest possible scope of semiconductor choices. Based on data for “real” cells and optical components, Rainbow was expected in 1997 to convert over 40% of incident solar energy to electricity at the system level.
To the knowledge of the present inventor, this concept has never come to a closure, presumably due to extreme difficulties encountered with the associated optics. In addition, this space system would only have a concentration ratio of a maximum of 20×, i.e., much lower than the 500× or more to reduce the effective cell cost dramatically. A thorough literature search has shown that in the past, the very promising method of spectral splitting and simultaneous use of discrete solar cells with different bandgaps has never reached its potential capacity and the technology was never exploited fully. The parent application to the present application represents a straight-forward approach to achieve break-through performance levels and with it to rapidly lower the cost of solar energy to competitive levels.
To address the large demand for noise-free and safe power transmission, without the use of electrical wiring, several new technologies are being introduced. The two major approaches are: (1) microwave and millimeter wave beaming and (2) optical fiber light transmission in conjunction with optically powered, sensors, transducers and data communications equipment. At the receiving end, microwaves and millimeter waves are converted into electricity via highly tuned phased array antennas. In the case of optical fiber power transmission, the conversion of light into electricity happens via a photovoltaic power converter, which is basically a slightly modified solar cell.
The conversion of beamed microwaves and millimeter waves into electric power is highly efficient. However, concerns with the potential hazardous impact of high intensity beams and the strong beam divergence limit the area of applicability of such power-beaming technologies to high altitudes and space. Optical fiber power transmission is distance- and power-limited due to optical absorption in the fiber and light input/output coupling losses. Most of the reported fiber optics power transfer applications are limited to local area networks (<<1 km) of power levels less than 1 watt and for the most a few microwatts. Thus, there is a need for a power beaming technology that can provide a wireless electric power source ranging from 1 watt to tens of kWatts and can be beamed from, say, 10 meters to several kilometers and beyond. Such high laser power levels are now available, due to emerging laser technologies such as chemical oxygen-iodine lasers (COIL) that are scalable up to 40 kW at a wavelength of 1.315 microns.
More recently, proposals have been made to convert coherent light to electricity. Such applications have been termed. “Laser Power Beaming” (LPB). LPB technology uses the properties of coherent light to transfer power between two locations without the need of any material or man-made medium. Thus, LPB is extremely fast and weightless. Over the last decade, total energy efficiencies for some lasers have improved significantly (40% and up) and reliable operation of high power lasers over long periods of time has been demonstrated in real life applications. The most efficient method of converting beamed laser power into electricity at the receiving end is the use of photovoltaic (PV) cells. As a result of recent research and development efforts on solar PV cell technology, solar-to-electricity conversion efficiencies as high as 36% has been achieved at 500×AM1.5 suns or about 50 W/cm2. Efficiencies for monochromatic light, as it is the case with LPB, are expected to be much higher. Research efforts in the field of thermo-photovoltaics (TPV) made it possible to develop new photovoltaic materials that are responsive in the near infrared range of the electromagnetic spectrum, that would, for example, operate at 40 to 45% efficiency at 1.315 wave length of the COIL lasers mentioned above. Such a TPV cell, for example, GaInAsSb/AlGaAs, can be used effectively with the COIL lasers mentioned above.
As an aside, it is important to note that in the past, integrating sphere systems have been used to measure, control, and monitor laser and laser diodes. However, to the knowledge of the inventor, the PowerSphere approach disclosed and claimed herein is the first disclosure that teaches how the integrating sphere concept can be exploited to convert beamed laser energy into electric power.